The present invention relates to an optical amplifying glass which is suitable for amplification of a light with a wavelength of from 1,530 to 1,630 nm.
As an optical communication system capable of coping with diversification of communication services, a wavelength division multiplexing communication system (WDM) has, for example, been proposed to increase the transmission capacity by increasing the number of wavelength division multiplexing channels.
In e.g. WDM using a light in C band (wavelength: 1,530 to 1,560 nm) or L band (wavelength: 1,570 to 1,620 nm) as a signal light, an optical fiber amplifier is essential which amplifies such a signal light. As such an amplifier, EDFA is being developed.
EDFA is an optical fiber amplifier wherein the core of the optical fiber is an Er-doped glass. As such an optical fiber, an Er-doped SiO2 fiber wherein the core is SiO2 glass, or an Er-doped fluoride glass fiber wherein the core is fluoride glass, may, for example, be mentioned.
However, EDFA employing the Er-doped SiO2 fiber has had a problem that the length of the optical fiber is typically at least 20 m, and it has to be wound into a bobbin in order to accommodate it in an EDFA container having a size of about 30 cm.
Whereas, EDFA using an Er-doped fluoride glass fiber, has had a drawback that its glass transition point is typically at most 320xc2x0 C., whereby a thermal damage is likely to result if the intensity of excitation light for optical amplification increases.
In order to solve these problems, JP-A-2001-102661 discloses a resin-coated glass fiber having a length of 6 cm, whereby a gain of at least 9 dB can be obtained with respect to a signal light having a wavelength of from 1.50 xcexcm to 1.59 xcexcm and an intensity of 0.001 mW. Here, the length is 6 cm, whereby it is unnecessary to wind it into a bobbin. Further, the core of the above resin-coated glass fiber, is an Er-doped bismuth oxide type glass (hereinafter referred to as the conventional glass) having Er doped in a proportion of 0.6 part by mass per 100 parts by mass of a bismuth oxide type matrix glass (as represented by mol %, Bi2O3: 42.8%, B2O3: 28.5%, SiO2: 14.3%, Ga2O3: 7.1%, Al2O3: 7.1%, and CeO2: 0.2%).
The above-mentioned gain of the conventional glass is one with respect to a case where the fiber length is 6 cm, and the intensity of the signal light is 0.001 mW. It is commonly known that the gain decreases as the intensity of the signal light increases, and a gain of a desired level may not be obtained by the conventional glass with respect to a signal light having an intensity of about 0.1 mW, which is commonly used for e.g. WDM.
Further, if the conventional glass is used for amplification of a light within a wavelength region including L band, a conversion efficiency of a desired level, may not be obtained.
It is an object of the present invention to provide an optical amplifying glass and an optical waveguide, which are capable of solving the above-mentioned problems.
The present invention provides an optical amplifying glass comprising 100 parts by mass of a matrix glass and from 0.1 to 10 parts by mass of Er doped to the matrix glass, wherein the matrix glass comprises Bi2O3, at least one of B2O3 and SiO2, at least one member selected from the group consisting of Ga2O3, WO3 and TeO2, and La2O3 in such a ratio that Bi2O3 is from 20 to 80 mol %, B2O3+SiO2 is from 5 to 75 mol %, Ga2O3+WO3+TeO2 is from 0.1 to 35 mol %, and La2O3 is from 0.01 to 15 mol %.
Further, the present invention provides an optical waveguide comprising the above optical amplifying glass as the core.
Now, the present invention will be described in detail with reference to the preferred embodiments.
The optical amplifying glass of the present invention (hereinafter referred to as the glass of the present invention) is used usually as a core of an optical waveguide having a core/cladding structure, such as a glass fiber having the same structure or a plane waveguide having the same structure. Such an optical waveguide is the waveguide of the present invention.
The optical waveguide of the present invention is suitable for amplifying a light with a wavelength of from 1,530 to 1,630 nm, particularly a light in C band in a short length. Further, it is suitable for amplifying a light in L band at a high conversion efficiency.
Such amplification is carried out by introducing an excitation light together with a light to be amplified (i.e. a signal light) into the core. As such an excitation light, a laser beam having a wavelength of from 970 to 990 nm or from 1,470 to 1,490 nm is usually employed. Usually, for amplification of a light in C band, an excitation light having a wavelength of from 970 to 990 nm is used, and for amplification of a light in L band, an excitation light having a wavelength of 1,470 to 1,490 nm is used. However, the excitation light is not limited thereto.
In a case where the optical waveguide of the present invention is used for amplification of a light in C band in a length of 8 cm or less, the gain for a light having a wavelength of from 1,530 nm to 1,560 nm and an intensity of 0.1 mW, is preferably at least 8 dB when the length of the optical waveguide is 5 cm. If the gain when the length is 5 cm, is less than 8 dB, the above problem may not be solved. Namely, when the length is 8 cm or less, no adequate gain may be obtained with respect to a signal light having an intensity of 0.1 mW. The above gain when the length is 5 cm, is more preferably at least 9 dB.
In a case where the optical waveguide of the present invention is used for amplification of a light in the wavelength region including L band, the conversion efficiency xcex7 for a light having a wavelength of 1,600 nm, is preferably at least 10%. If it is less than 10%, the desired gain may not be obtained. It is more preferably at least 15%. Here, xcex7 is represented by a percentage of the ratio of the signal light output intensity to the excitation light intensity.
It is more preferred that xcex7 is at least 10%, and the 3-dB down bandwidth for a light having an intensity of 1 mW within a wavelength region of from 1,530 to 1,620 nm, is at least 55 nm, particularly preferably at least 60 nm.
Further, it is more preferred that xcex7 is at least 10%, and the gain for a light having a wavelength of 1,620 nm and an intensity of 1 mW, is at least 10 dB, particularly preferably at least 15 dB.
It is particularly preferred that xcex7 is at least 10%, the 3-dB down bandwidth for a light having an intensity of 1 mW within a wavelength region of from 1,530 to 1,620 nm, is at least 55 nm, and the gain for a light having a wavelength of 1,620 nm and an intensity of 1 mW, is at least 10 dB.
In an optical fiber having the glass of the present invention as a core (hereinafter referred to as an optical fiber of the present invention), the core diameter and the cladding diameter are typically from 2 to 10 xcexcm, and from 100 to 200 xcexcm, respectively.
When the optical fiber of the present invention is used for EDFA without being wound into a bobbin, its length is preferably at most 8 cm, more preferably at most 6 cm, particularly preferably at most 5 cm.
It is preferred that the refractive index n2 of the cladding of the optical fiber of the present invention and the refractive index n1 of the core i.e. the glass of the present invention, will satisfy the following formula. Here, n1 is typically from 1.8 to 2.2.
0.0005xe2x89xa6(n1xe2x88x92n2)/n1xe2x89xa60.1
Further, it is preferred that the above cladding is made of a glass, and it is more preferred that such a glass consists essentially of, as represented by mol %, from 25 to 70% of Bi2O3, from 5 to 74.89% of B2O3+SiO2, from 0.1 to 30% of Al2O3+Ga2O3, and from 0.01 to 10% of CeO2.
The optical fiber of the present invention may be prepared, for example, by preparing a preform having the core glass and the cladding glass combined by a well-known extrusion method, and drawing such a preform.
The glass transition point Tg of the glass of the present invention is preferably at least 360xc2x0 C. If Tg is lower than 360xc2x0 C., when a laser beam with a high intensity is used as an excitation light, the temperature of the glass tends to be locally high and thermally damaged, whereby the optical loss may increase, and the optical amplification tends to be inadequate. More preferably, the glass transition point is at least 400xc2x0 C., particularly preferably at least 420xc2x0 C.
The glass of the present invention comprises a matrix glass and Er.
If the amount of Er to the matrix glass, is less than 0.1 part by mass per 100 parts by mass of the matrix glass, no adequate gain can be obtained. It is preferably at least 0.2 part by mass. If it exceeds 10 parts by mass, vitrification tends to be difficult, or due to concentration quenching, the gain tends to rather decrease. It is preferably at most 7 parts by mass, more preferably at most 4 parts by mass, particularly preferably at most 3 parts by mass.
In a case where the glass of the present invention is to be used as an optical fiber (the length is typically at most 8 cm) to be used for EDFA without being wound into a bobbin or to be used for a compact plane waveguide (the length is typically at most 8 cm) to be used for EDFA, Er is preferably incorporated in an amount of at least 0.5 part by mass, more preferably at least 0.8 part by mass, particularly preferably at least 1.0 part by mass, per 100 parts by mass of the matrix glass.
The proportion of Er in these cases, is preferably from 1 to 3 parts by mass, more preferably from 1.2 to 3 parts by mass, particularly preferably from 1.5 to 3 parts by mass, per 100 parts by mass of the matrix glass.
In a case where the glass of the present invention is to be used for amplification of a light in a wavelength region including L band, Er is incorporated preferably in an amount of at least 0.1 part by mass and less than 1 part by mass, per 100 parts by mass of the matrix glass. It is more preferably at least 0.2 part by mass, particularly preferably at least 0.3 part by mass, and it is more preferably at most 0.9 part by mass, particularly preferably at most 0.8 part by mass.
Now, the composition of the matrix glass in the glass of the present invention will be described, wherein mol % will be represented simply by %.
Bi2O3 is an essential component. If its content is less than 20%, the wavelength width xcex94xcex wherein the gain is obtainable, tends to be small. Its content is preferably at least 30%, more preferably at least 35%, particularly preferably at least 40%. If the content exceeds 80%, vitrification tends to be difficult, devitrification tends to take place during processing into a fiber, or Tg tends to be too low. The content is preferably at most 70%, more preferably at most 60%, particularly preferably at most 50%. Here, devitrification is one where precipitation of crystals is remarkable and one whereby fiber breakage is likely to take place during processing into a fiber, or fiber breakage is likely to take place during the use as an optical fiber.
B2O3 and SiO2 are network formers, and at least one of them must be contained in order to facilitate formation of glass by suppressing precipitation of crystals during the production of the glass. If their total content B2O3+SiO2 is less than 5%, vitrification tends to be difficult, or devitrification takes place during processing into a fiber. The total content is more preferably at least 10%, more preferably at least 15%, particularly preferably at least 19%, most preferably at least 25%. If it exceeds 75%, the gain tends to decrease. It is more preferably at most 60%, still more preferably at most 55%, particularly preferably at most 45%, most preferably at most 40%.
The content of B2O3 is preferably at most 60%, more preferably at most 45%, particular preferably at most 30%. In a case where it is desired to improve the water resistance or to increase the gain, the content is preferably adjusted to be at most 20%, and more preferably, no B2O3 is incorporated.
The content of SiO2 is preferably at most 60%, more preferably at most 50%, particularly preferably at most 45%, most preferably at most 40%. When SiO2 is incorporated, its content is preferably at least 1%, more preferably at least 10%, particularly preferably at least 19%, most preferably at least 25%.
Ga2O3, WO3 and TeO2 are components which increase xcex94xcex. At least one member from these three components must be incorporated. If their total content Ga2O3+WO3+TeO2 is less than 0.1%, xcex94xcex tends to be small. The total content is preferably at least 3%, more preferably at least 5%, particularly preferably at least 10%, and if it exceeds 35%, the gain tends to be low. It is preferably at most 30%, more preferably at most 25%.
When it is desired to increase xcex94xcex, it is preferred to incorporate Ga2O3.
The content of Ga2O3 is preferably at most 30%, more preferably at most 20%. When Ga2O3 is incorporated, its content is preferably at least 1%, more preferably at least 5%, particularly preferably at least 10%.
The content of WO3 is preferably at most 30%, more preferably at most 20%, particularly preferably at most 10%. When WO3 is incorporated, its content is preferably at least 1%, more preferably at least 3%.
The content of TeO2 is preferably at most 30%, more preferably at most 20%. When TeO2 is incorporated, its content is preferably at least 1%, more preferably at least 3%.
La2O3 is essential and has an effect to make concentration quenching less likely to take place or an effect to increase the gain. If it is less than 0.01%, such effects tend to be small. It is preferably at least 0.1%. If it exceeds 15%, vitrification tends to be difficult, or the optical loss tends to increase. It is preferably at most 12%, more preferably at most 10%.
In a case where the glass of the present invention is to be used for an optical fiber to be used for EDFA without being wound into a bobbin, or to be used for a compact plane waveguide to be used for EDFA, the amount of Er incorporated to the matrix glass tends to be typically high at a level of at least 1 part by mass, and in order to suppress concentration quenching due to Er, La2O3 is preferably at least 1%, more preferably at least 2%.
In a case where the glass of the present invention is to be used for amplification of a light in a wavelength region including L band, La2O3 is preferably from 0.5 to 4%. In such a case, the length of the optical fiber or the plane waveguide is long (typically at least 80 cm), and if it exceeds 4%, the desired optical amplification tends to be hardly obtainable due to an optical loss. It is more preferably at most 3%, particularly preferably at most 2.5%.
In the present invention, the matrix glass preferably consists essentially of, based on the following oxides:
Of the above preferred matrix glass, components other than Bi2O3, B2O3, SiO2, Ga2O3, WO3, TeO2 and La2O3 as described in the foregoing, will be described.
Al2O3 is not essential, but may be incorporated up to 10%, to facilitate formation of the glass by suppressing precipitation of crystals during the preparation of the glass. If it exceeds 10%, the optical amplification factor may decrease. It is more preferably at most 9%, still more preferably at most 8%, particularly preferably at most 7%, most preferably at most 5%. When Al2O3 is incorporated, its content is preferably at least 0.1%, more preferably at least 1%, particularly preferably at least 2%.
In order to facilitate formation of the glass by suppressing precipitation of crystals during the preparation of the glass, it is preferred to incorporate at least one of Al2O3 and Ga2O3, and their total content Al2O3+Ga2O3 is preferably at most 30%. If the total content exceeds 30%, vitrification tends to be difficult, or Tg tends to be low. More preferably, it is at most 25%. Further, Al2O3+Ga2O3 is preferably at least 3%, more preferably at least 8%, particularly preferably at least 12%.
GeO2 is not essential, but may be incorporated up to 30%, as it has an effect to facilitate formation of the glass or an effect to increase the refractive index. If it exceeds 30%, the glass tends to crystallize. It is preferably at most 10%, more preferably at most 5%. When GeO2 is incorporated, its content is preferably at least 0.1%, more preferably at least 1%.
CeO2 is not essential, but may be incorporated up to 2% to prevent Bi2O3 from precipitating in the form of metal bismuth in the molten glass and thereby lowering the transparency of the glass. If it exceeds 2%, coloration of the glass with a yellow color or an orange color tends to be substantial, whereby the transmittance tends to decrease. It is preferably at most 1%, more preferably at most 0.5%. When CeO2 is incorporated, its content is preferably at least 0.1%. When it is desired to increase the transparency, it is preferred that no CeO2 is incorporated.
Each of TiO2 and SnO2 is not essential, but each may be incorporated within a range of up to 30%, in order to suppress devitrification during the processing into a fiber. Each content is more preferably at most 10%.
A preferred matrix glass in the present invention consists essentially of the above-described components, but other components may be incorporated within a range not to impair the purpose of the present invention. The total content of such xe2x80x9cother componentsxe2x80x9d is preferably at most 10%. For example, MgO, CaO, SrO, BaO, Na2O, K2O, ZrO2, ZnO, CdO, In2O3, PbO, etc., may be incorporated to suppress devitrification during the processing into a fiber or to facilitate vitrification, and Yb2O3 may be incorporated to suppress devitrification or concentration quenching. Further, when Yb2O3 is incorporated, its content is preferably at most 5%.
With respect to the method for producing the glass of the present invention, there is no particular restriction. For example, it may be produced by a melting method wherein the starting materials are prepared and mixed, put into a gold crucible, an alumina crucible, a SiO2 crucible or an iridium crucible and melted at a temperature of from 800 to 1,300xc2x0 C. in air, and the obtained melt is cast into a prescribed mold. Otherwise, it may be produced by a method other than the melting method, such as a sol-gel method or a gas phase vapor deposition method.